The invention relates generally to the cooling of internal combustion engines and, more particularly to pressure controlled cooling of marine engines. Prior cooling systems are disclosed in the pending U.S. application Ser. No. 444,049, now U.S. Pat. No. 3,908,579, filed Feb. 20, 1974, and in the Kueny U.S. Pat. No. 3,667,431 issued June 6, 1972.